Double stop structure for a pressure transducer

ABSTRACT

A stop member is secured to a piezoresistive semiconductor bossed diaphragm at the peripheral area, and includes a first and second slotted apertures in communication with the central active area, the first and second slotted apertures correspond in location with opposing sides of a central boss. The stop member includes a stop cavity located between the first slotted aperture and the second slotted aperture, and the stop cavity overlies the central boss and is separated therefrom to enable the diaphragm to deflect when a force is applied and to enable the central boss to impinge on the surface of the stop cavity when an excessive force is applied. The first and second slotted apertures permit another force to be applied to the active region of the diaphragm in a direction opposite to the stopped direction. A second stop member is secured to the diaphragm to provide stopping in either direction.

This application is a divisional of U.S. patent application Ser. No.10/057,130, filed Oct. 24, 2001, now U.S. Pat. No. 6,588,251.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to semiconductor transducers,and more particularly to differential pressure transducers suitable formeasuring differential pressure that employ piezoresistive sensors on abossed diaphragm and stop members for limiting the deflection of thediaphragm upon the application of excessive force, and methods of makingthe same.

2. Description of Related Background Art

Semiconductor pressure transducers are employed in the measurement ofpressure in numerous types of applications. Many pressure transducersemploy a relatively thin diaphragm or deflecting member fabricated fromsemiconductor material such as silicon. The diaphragm is the thinportion of the transducer, which determines the pressure range of thedevice, which varies depending upon the thickness of the diaphragm.Typically upon the diaphragm is deposited or diffused a piezoresistivestrain gage configuration, such as a bridge circuit, whereby theresistors associated with the bridge exhibit a change in resistanceaccording to a deflection in the diaphragm as is well known. Bymonitoring the change in resistance, one obtains an output voltageindicative of the applied pressure or force.

One particular type of pressure transducer is a differential pressuretransducer. Differential pressure transducers provide an output which isthe difference between two pressures. For example, when a pressure P₁ isapplied to one face of the deflecting member and a pressure P₂ isapplied to the other face of the deflecting member, the resultingdeflection will be determined by the difference in pressure, i.e.,P₁-P₂. An example of differential piezoresistive bridge pressuretransducers is illustrated in U.S. Pat. No. 6,272,928, entitled“Hermetically Sealed Absolute and Differential Pressure Transducer,”assigned to the assignee herein.

Frequently, differential pressure transducers are subjected to a high“line” pressure. This high “line” pressure refers to the pressure whichboth sides of the sensor are subjected to while simultaneously measuringthe difference in pressure from one side of the sensor to the other.Often the differential pressure is much smaller than the “line”pressure. In many instances, however, due to blockage in the line, thefull “line” pressure applied to either side of the sensor separatelythereby creates an enormous pressure difference across the sensor. Inthese instances, the deflection of the sensor in either direction mustbe restrained to avoid excessive strain on the diaphragm which causes itto fracture.

One known method of limiting the deflection of the sensor is to attach adeflection limiting member with a shallow cavity to the sensor, commonlyreferred to as a “stop.” The cavity depth is fixed to limit thedeflection of the sensor a predetermined distance to thereby avoidexcessive strain of the sensor. When a stop disposed on a sensorstructure has no aperture to permit the passage of gas or air, the stopwill only permit deflection until the sensor contacts the bottom of thecavity of the stop. In measuring an absolute pressure (unidirectional),it is apparent that this type of structure will limit the deflection ofthe sensor to insure that above a certain pressure, the sensor cannotdeflect, thus preventing excessive strain on the sensor.

For bi-directional sensors, a stop without an aperture cannot be used.For the sensor to respond to pressure from either direction, the stopsrequire apertures to allow the applied pressure to reach the deflectingsensor structure thereby causing it to deflect. Prior to the presentinvention, apertures have been located in the central portion of stopstructures. To ensure adequate pressure application to the sensor, thecentral aperture in the stops should be as large as possible. To ensurethe best stopping, however, the apertures in the stop should be as smallas possible. When the apertures are disposed in the central portion ofthe stop structure, the region of the sensor where the most extensivedeflection occurs does not entirely contact the bottom of the cavity ofthe stop where the aperture is located. Clearly, there are problemsassociated with stops having a central aperture, which the presentinvention seeks to avoid.

There still remains a need for a stop structure that provides improvedstopping capabilities, while ensuring adequate pressure application tothe sensor structure. There also remains a need for a stop structurethat provides apertures that are large enough to insure adequatepressure application, while providing apertures that are small enough toinsure the best “stopping.” There also remains a need for asemiconductor sensor that measures differential pressure across thesensor while being capable of withstanding high unidirectional pressureacross the sensor.

SUMMARY OF THE INVENTION

Briefly described, a preferred embodiment of the present inventionprovides a semiconductor sensor for measuring differential pressureacross the sensor diaphragm and operable in the presence of an excessunidirectional force applied to the sensor from either direction to stopsaid sensor diaphragm from further deflection during the presence ofsaid excess force which tends to fracture said diaphragm. The transducersensor includes a planar semiconductor diaphragm including a centralactive area on a top surface surrounded by a groove of a given width anddepth which forms a central boss, the central area within the groovecapable of deflection, and a peripheral non-active area surrounding thegroove. The diaphragm has a relatively smooth bottom surface upon whichpiezoresistive sensors are formed that correspond in location on thebottom surface with the groove on opposing sides of the central boss ofthe top surface. A stop member is secured to the diaphragm at theperipheral area, and includes a first slotted aperture and a secondslotted aperture in communication with the active area, the first andsecond slotted apertures extending generally along the length of theactive area and which correspond in location with opposing sides of thecentral boss. The stop member includes a stop cavity located between thefirst and second slotted aperture, and the stop cavity overlies thecentral boss and is separated therefrom to enable the diaphragm todeflect when a force is applied and to enable the central boss toimpinge on the surface of the stop cavity when an excessive force isapplied. The first and second slotted apertures permit another force tobe applied to the active region of the diaphragm in a direction oppositeto the stopped direction.

In an alternate preferred embodiment of the invention, a stop member fora differential semiconductor sensor employing a bossed diaphragmincludes a planar member having a first and a second slotted aperture,the first the second slotted apertures being substantially parallel andextending through the planar member. The first and the second slottedapertures are positioned so that when the stop member is disposed on abossed diaphragm the first and the second slotted apertures are locatedalong the outer edge of the active region of the diaphragm, and theslotted apertures are relatively as long as the active region. The stopmember includes a stop cavity located between the slotted apertures, toallow a central boss of the diaphragm to impinge upon the surface of thestop cavity when an excessive force is applied to the diaphragm.

In an alternative preferred embodiment, the invention includes a doublestop structure for a semiconductor pressure transducer employing abossed diaphragm, including a first stop member that includes aplurality of cutout portions on the periphery thereof for accessingperipheral contact areas of a bossed diaphragm of a pressure transducer;a first cavity in a central portion of the first stop member forreceiving the diaphragm of a pressure transducer; and two slottedapertures adjacent the first cavity and extending through the first stopmember permitting access to the environment, and a second stop memberincluding: a second cavity formed in a central portion of the secondstop member for receiving the diaphragm, and two slotted aperturesadjacent the second cavity and extending through said second stop memberpermitting access to the environment. The first stop and second stopmember are operable under excessive force to prevent the diaphragm fromexcessive strain leading to fracture.

The present invention also provides a preferred method for manufacturinga differential pressure transducer that includes the steps offabricating a bossed diaphragm including a first substrate composed ofsilicon, and a second substrate composed of silicon dioxide, thediaphragm including an central active region, and a peripheralnon-active region separated by a groove which defines a central bosscapable of deflecting, disposing a plurality of piezoresistive sensingelements on the active region, forming a plurality of contact areas onthe diaphragm that extend from the sensing elements to the peripheralregion, forming a first stop member including a cavity in the centralportion thereof, a plurality of cutout portions of the peripherythereof, and two slotted apertures extending through the first stopmember, and forming a second stop member including a cavity in thecentral portion thereof and two substantially parallel slotted aperturesextending through the second stop member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of the diaphragm of the transducer of thepresent invention.

FIG. 2 is a cross sectional side view of the diaphragm of FIG. 1.

FIG. 3 is a top plan view of a first stop member according to apreferred embodiment of the present invention.

FIG. 4 is a cross sectional side view of the first stop member of FIG.3.

FIG. 5 is a top plan view of a second stop member according to apreferred embodiment of the present invention.

FIG. 6 is a cross sectional side view of the second stop member of FIG.5.

FIG. 7 is an exploded view of the transducer of the present invention.

FIG. 8 is a cross sectional side view of the transducer according to apreferred embodiment of the present invention.

FIG. 9 is an isometric view of the sensor of FIGS. 7 and 8.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the Figures, it is to be understood that like numbersidentify like elements of the invention. Referring to FIG. 1, there isshown a top plan view of a diaphragm 10 of the transducer sensoraccording to a preferred embodiment of the present invention. Thediaphragm 10 with a “bossed” central area is constructed in accordancewith the disclosure of U.S. Pat. No. 4,236,137 which is commonlyassigned to the assignee herein, the entire disclosure of which ishereby incorporated by reference. The bossed diaphragm disclosed in U.S.Pat. No. 4,236,137 is particularly well suited for the preferredembodiment of the present invention. The sensor includes a sensingdiaphragm and a picture frame type of clamping region. The diaphragm hasa thin part, the thickness of which controls the pressure range of thedevice. The boss is a thicker region located in the middle of thediaphragm.

As illustrated in FIG. 1, the diaphragm 10 is rectangular or square inconfiguration. Piezoresistive sensors 20 are disposed on the diaphragm10 in the central area 16 of the diaphragm on a relatively smooth bottomsurface 14, typically by diffusion or deposition. Contact areas 22extend from the piezoresistive sensors 20 to the peripheral area 18 ofthe diaphragm 10. The piezoresistive sensors may be configured as aWheatstone bridge circuit.

Referring to FIG. 2, the cross sectional side view of the diaphragm 10of FIG. 1 illustrates a first substrate 12 composed of silicon, and asecond substrate 14 composed of silicon dioxide. There is also a groove11 formed by an etching process with a selected width and depth whichsurrounds a central active area of the diaphragm 10, which forms acentral boss 24 on the top surface 13 of the diaphragm 10. The thicknessof the diaphragm 10 controls the pressure range of the sensor device,and the central boss 24 serves to stiffen the diaphragm 10 to improveperformance and linearize the mechanical response of the sensor. For atypical piezoresistive sensor diaphragm having a deflecting portion of0.06″×0.060″ to about 0.03″×0.03″, the deflection required to obtainsufficient stress in the sensor to obtain the proper piezoresistiveoutput from the sensor is approximately 0.0001″, while the deflection ofthe sensor sufficient to cause excessive strain, which leads tofracture, is approximately 0.0003″. For the “clamped edge” sensorparticularly suited for the present invention and described in U.S. Pat.No. 4,236,137, the maximum deflection occurs at the center of thedeflecting member or bossed diaphragm. The clamped edge allows thetransducer to be clamped or mounted to a housing at the surface thereof.

Still referring to FIG. 2, the peripheral area 18 surrounding the groove11 and external to the active area 16 is also referred to as anon-active area, and serves as a “picture frame” type of clampingregion. As illustrated in FIGS. 1 and 2, the contact areas 20 extend tothe non-active or peripheral area 18. Leads 26 are disposed at thedistal ends of the contact areas 20. Leads are added at any convenientstage in the manufacturing process. Leads are located on the inactive orperipheral area 18 of the diaphragm 10 and being isolated do not affectthe overall response of the diaphragm 10. As further illustrated in FIG.2, piezoresistive sensors 20 are disposed in the active area 16 of thediaphragm 10 on the bottom surface 14 in a location that corresponds tothe location of the groove 11 on the top surface 13, and are disposed oneither side of the groove 11 on opposing sides of the central boss 24.

Referring now to FIG. 3, there is illustrated a top plan view of a firststop member 30 according to a preferred embodiment of the presentinvention. The stop member is preferably fabricated from glass or aninsulator. The periphery of the inner surface 31 of the first stopmember 30 is secured to the diaphragm 10 at the peripheral area 18. Apair of slotted apertures 36 are formed in the first stop member 30which communicate pressure media to the active area 16 of the diaphragm10. The slotted apertures 36 extend generally along the length of theactive area 16 but are sufficiently narrow to allow the boss 24 toimpinge on the bottom of the cavity 34 in the stop 30. When the firststop member 30 is disposed on the diaphragm 10, the location of theslotted apertures 36 coincides with the location of the groove 11 onopposing sides of the central boss 24.

Slotted apertures 36 are substantially parallel to each other, and areapproximately equidistant from central axis X of the stop member 30. Theslotted apertures 36 are rectangular in configuration, and extendthrough the stop member 30 from an inner surface 31 to an outer surface32 of the stop member 30. Although slotted apertures 36 are illustratedas having a rectangular configuration, other suitable geometric shapesmay be used provided that the apertures are large enough to permitapplied pressure to the diaphragm but small enough to insure a goodstopping surface. For a typical sensor structure with a deflecting areaof 0.06×0.06 inches, a preferred slot width is 0.01 inch and preferredlength is 0.06 inch, while the dimensions of the boss 24 are 0.04×0.04inches.

Disposed between the slotted apertures 36 is a stop cavity 34, which isformed by an etching process. The stop cavity 34 overlies and isseparated from the central boss 24 to enable the diaphragm 10 to deflectwhen a force is applied to the sensor. The stop cavity 34 also enablesthe diaphragm 10 to impinge upon the surface of the stop cavity 34 whenexcessive force is applied, i.e., a pressure above the selected pressurerange of the device, to avoid fracture to the thin diaphragm 10. Thefirst stop member 30 also includes a plurality of cutout portions 33 asindicated by the dashed lines in FIG. 3. The cutout portions 33 permitaccess to the contact areas 22 and to any leads 26 disposed thereon.

Referring to FIG. 4, which illustrates a cross sectional side view ofthe first stop member of FIG. 3, the stop cavity 34, cutout portions 33and the slotted apertures 36 are further illustrated. A preferred cavitydepth is approximately 0.0002 to 0.0003 inches and width is 0.04 inches.In this preferred embodiment, two 0.01×0.06 inch slotted apertures, anda cavity depth of 0.0002 to 0.0003 inches, the boss 24 can impinge onthe bottom of the cavity 34 when high pressures are applied.

Referring to FIGS. 5 and 6, a second stop member 40 is illustrated. Ascan be seen, the second stop member 40 is similar in configuration tothe first stop member 30. As with the first stop member 30, the secondstop member 40 includes two slotted apertures 46 which communicate withthe active area 16 of the diaphragm 10. The slotted apertures 46 extendthrough the second stop member 40 from an inner surface 41 to an outersurface 42 of the stop member 40, and are substantially parallel to eachother, and are approximately equidistant from central axis X of the stopmember 30. The stop cavity 44 formed by an etching process is locatedbetween the slotted apertures 46, and is approximately 0.0002 to 0.0003inches in depth. The periphery of the inner surface 41 of the secondstop member 40 is attached to the peripheral region 18 of the diaphragm10.

Referring to FIG. 6, the stop cavity 44 and the slots 46 are furtherillustrated in a cross sectional view. The first and second stop members30 and 40 are composed of a non-conductive borosilicate glass with a lowcoefficient of expansion, commonly known as Pyrex. The slotted apertures36 and 46 illustrated in FIGS. 3 to 6 are substantially rectangular inconfiguration, but other suitable geometric shapes may be used, providedthe apertures are large enough to permit proper passage of pressuremedia, but narrow enough to permit the entire bottom surface of the boss24 to touch the bottom of the cavity.

Referring to FIG. 7, an exploded view of the transducer semiconductorsensor including first and second stop members 30 and 40 is illustrated.As can be seen, the slotted apertures 36 of the first stop member 30 areopen to the environment and visible on the outer surface 32. The innersurface 31 of the first stop member 30 is electrostatically bonded onthe surface 14 of diaphragm 10 at the peripheral area 18, and the cutoutportions 33 provide access to the contact areas 22. The inner surface 41of a second stop member 40 is electrostatically bonded on an oppositesurface 13 of the diaphragm 10 at the peripheral area 18 when stoppingis desired in either direction.

Referring to FIG. 8, a cross sectional side view of the transducer ofFIG. 7 illustrates a preferred embodiment of the present invention as acomposite structure. As can be seen, slotted apertures 46 are providedfor supplying pressure media to the diaphragm 10. Shallow cavity 44serves to communicate the pressure received to the active area 16 of thediaphragm. Likewise, slotted apertures 36 permit pressure media into thecavity 34, which in turn serves to communicate a pressure received tothe diaphragm 10. The applied pressure or pressures serve to deflect thesensor (the deflection being caused by differences of oppositely appliedpressures indicates the differential pressure) and a signal indicativeof the differential pressure can be supplied using the sensor and anelectrical connection to the contact areas. The cavity 44 of stop member40 also limits the deflection of the sensor as it is shallow enough topermit the boss 24 to impinge upon the surface thereof when an appliedforce is excessively high.

Referring to FIG. 9, an isometric view of the invention furtherillustrates the transducer 50 of the present invention with a doublestop structure disposed thereon. In FIG. 9, the piezoresistive sensors20 are located within the active region 16 of the diaphragm 10 on thebottom surface 14. The location of the sensors 20 correspond to thelocation of the groove 11 on the top surface 13 of the diaphragm 10,and, as also illustrated in FIG. 2, are disposed on either side of thegroove 11 on opposing sides of the central boss 24. The location of theslotted apertures are also seen in FIG. 9 as bording the piezoresistivesensors 20. The slotted apertures 36 and 46, the piezoresistive sensors20, and the groove 11 coincide in spatial placement in a givendirection.

The transducer 50 in operation will now be described. When pressure isapplied to the sensor through the slotted apertures 36, the bosseddiaphragm 10 begins to deflect and the piezoresistive sensors 20 convertthe mechanical signal into electrical output. When high pressures areapplied, the entire bottom surface of the central boss 24 can rest uponthe bottom of the cavity 34 while the slotted apertures 36 continue toadmit the pressure media through the slotted apertures 36. This permitsanother force to the active region in a direction opposite to thestopped direction. When stopping in both directions is desired, a secondstop structure 40 with similar slotted apertures 46 is affixed to theside of the sensor that does not have a piezoresistive sensor network.

Thus, the diaphragm 10 is provided with a double “stop” in eitherdirection, while still allowing pressure media to reach both sides ofthe sensor diaphragm 10. For example, if the thickness of the diaphragmis capable of withstanding and measuring a pressure of 4 psi as amaximum limit, and due to unidirectional or high “line” pressure orblockage in one of the pressure lines causing a spike of more than 4psi, the pressure received by the diaphragm is above the limit of 4 psi,the diaphragm can withstand the spike or overpressure, since cavities 34and 44 provides a “stop” surface which limits the further deflection ofthe diaphragm, and possible fracture. In the preferred embodiment shownand described, the cavity 44 is shallow enough to provide a surface uponwhich the central boss 24 impinges.

In addition to the foregoing advantages, the placement of the slottedapertures 34 and 44 equidistant from the center of the stop member 30and 40 and the selected dimensions of the cavities provides a surfaceupon which the diaphragm impinges, as opposed to having a cover or stopmember with a central aperture, in which the diaphragm would notcompletely impinge upon the surface in the area of the aperture wherethe diaphragm has the greatest amount of deflection.

Although the invention has been described and illustrated in thepreferred form with a certain degree of particularity, it is understoodthat the present disclosure of the preferred form has been made only byway of example, and that numerous changes in the details of constructionand combination and arrangement of parts may be made without departingfrom the spirit and scope of the invention as claimed herein. It isintended that the invention shall cover, by suitable expression in theappended claims, whatever features of patentable novelty exist in theinvention disclosed.

What is claimed is:
 1. A method of manufacturing a differential pressuretransducer, comprising: fabricating a bossed diaphragm comprising afirst substrate composed of silicon, and a second substrate composed ofsilicon dioxide, said diaphragm including an central active region, anda peripheral non-active region separated by a groove which defines acentral boss capable of deflecting; disposing a plurality ofpiezoresistive sensing elements on said active region; forming aplurality of contact areas on said diaphragm that extend from saidsensing elements to said peripheral region; and forming a first stopmember including a cavity in the central portion thereof, a plurality ofcutout portions of the periphery thereof, and two slotted aperturesextending through said stop member, and forming a second stop memberincluding a cavity in the central portion thereof and two substantiallyparallel slotted apertures extending through said second stop member,and placing said first stop member on the bossed surface of saiddiaphragm to enable said boss to impinge on said stop member for a forcein a first direction, and placing said second stop member on the surfaceopposite said boss to enable said diaphragm surface opposite said bossto impinge on said stop member for a force in a direction opposite tosaid first direction.
 2. The method according to claim 1 furtherfabricating a piezoresistive sensor array on said surface of saiddiaphragm opposite said boss.
 3. The method of claim 2, the steps offorming a first and a second stop member further comprising: formingsaid first and said second stop members of borosilicate glass.
 4. Themethod of claim 2, further comprising the step of electrostaticallybonding said first and said second stop members to said diaphragm.